Trigeminal Mediation of Mammalian Aversion to Insect Chemical Defense Compounds
Paige M. Richards, Annalyn M. Welp, Deirdre R. Craven and Wayne L. Silver
Department of Biology, Wake Forest University, Winston-Salem, NC
Background
One way that insects protect their territory or avoid predation is by releasing defensive chemicals. A wide variety of insects use the same chemicals for this purpose. We
hypothesize that defensive chemicals used by multiple insect orders target the mammalian trigeminal system and elicit chemesthesis. In the current study, we have begun to test
some of these compounds to determine if they irritate a mammalian predator (rat). We determined whether the chemicals activate the trigeminal nerve by recording from the
ethmoid branch while perfusing stimuli through the rat’s nasal cavity. As trigeminal nerve activation depresses respiratory frequency, we also monitored respiration in the rats both
before and after stimulus presentation. We then determined if rats are behaviorally averse to these compounds at concentrations released by insects.
Experimental Set-Up for Delivering Fluids to the Rat Nasal
Cavity and recording neural and respiratory responses
FIGURE 1. Insect defense chemicals activate the mammalian trigeminal nerve.
Representative
Releasing Insect
Stimulus
Ringer’s
(saline)
Representative Nerve
Representative
Recording
Respiratory Trace
Stimulus
Representative
Releasing Insect
Formic Acid (50
mM)
N/A
Representative Nerve
Representative
Recording
Respiratory Trace
Stimulus
Representative
Releasing Insect
M. gulosa
trans-2Hexenal
(10 mM)
2-Heptanone
(25 mM)
O. fasciatus
A. mellifera
S. sipylus
6-methyl-5hepten-2-one
(20 mM)
Benzoquinone
(75 mM)
B. explodens
Representative
Respiratory Trace
Tetradecane
(1.5 g/mL bolus)
F. rufa
Benzaldehyde
(50 mM)
Representative Nerve
Recording
trans-2-Hexen1-ol
(100 mM)
L. fuliginosis
P. apterus
Summary
FIGURE 2. Rats are behaviorally averse to defense chemicals at
concentrations insects release.
•
•
•
A. Image of a rat in the plexiglass arena for the
behavioral aversion paradigm. Either an irritant or
water is placed in one corner with water in all other
corners. The time the rat spends in each corner (or
zone) during a 10 minute period is determined using
Noldus Ethovision. Rats were habituated to the
arena for three days prior to testing.
•
We have identified several insect allomones that
activate the rat trigeminal nerve resulting in respiratory
depression.
Several of these compounds also elicit behavioral
aversion in rats at biologically relevant concentrations.
One tested allomone (tetradecane) did not elicit
trigeminal responses nor did it cause behavioral
aversion even at the highest concentration possible.
Three compounds – formic acid, 6-methyl-5-hepten-2one and trans-2-hexen-1-ol did not elicit behavioral
aversion at relevant concentrations even though they
activated the trigeminal nerve when tested.
Future Directions
•
B.
C.
•
Conduct calcium image studies on rat trigeminal
ganglion cultures.
Determine which sensory receptors these defense
compounds may be activating using physiological and
molecular techniques.
This project is funded by the Center for Molecular Communication and
Signaling at Wake Forest University.
Sample tracking for a rat in an arena containing (B) AITC in the bottom
right corner or (C) water in every corner..
D. Mean time spent by rats in a designated quadrant (either containing irritant
or water) vs. mean time spent in each of the other quadrants (all containing
only water). Asterisks indicate statistical significance (p<0.05). n=6-8
Insect Images are taken from the Encylopedia of Life (eol.org). Either
Public Domain or Creative Commons Licensing photographs by (in
order) : Pavla Tochorova, Ivo Antusek, Richard Bartz, California
Academy of Science (2 photos), Phil Myers, Adam Opiola